1993 Civic Hard Starting Problem

Jim Yanik · Sep 7, 2004

Jim said:

Well,now we have diodes that can take the back EMF,back then we
didn't,had to use big stacks of selenium wafers for rectifiers,and
they were SLOW(in turn-off time,critical for flyback operation).Your
PC power supply works just fine using diode snubbers in the flyback
mode supply.They even make the switching transistors with
on-substrate diodes for this purpose.
Click to expand...

jim, sorry if i'm coming across as unclear.

diodes on relays aren't used in normal forward conduct mode. think
about it: if they conducted the current used to energise the coil, the
coil would never activate. so they're oriented the opposite way.
where's the benefit? well, when the magnetic field energy of the
solenoid breaks down after switch-off, it changes rapidly. a rapidly
changing magnetic field induces an electric current in the coil that
just happens to be wrapped around it. depending on the number of
turns & rate of decay, that current can have many thousands of volts.
diodes break down and conduct reverse voltage at some level, and this
level of breakdown can be carefully designed. so, the diodes used to
protect relay circuits do this at some appropriate level - whatever -
and so the voltage spike never goes beyond this, hence protection.

the devices used in switch mode power supplies are a very different
creature. unlike simple full wave rectifiers that conduct the whole
waveform, switching devices need to be able to switch a forward
current in the middle of a wave. this means a very large momentary
power dissipation within the device, so to do this successfully,
[semiconductors are not great power dissipators] the switch needs to
be /fast/.
Click to expand...

But once the switching transistor turns OFF,there's still the flyback EMF
that has to be dealt with (on the primary side),otherwise the switching
xstr breaks down and fails due to those high voltages.That's where the fast
recovery diodes enter the picture.(along with other snubbing circuitry) The
fast diode allows less of the back EMF to reach the switcher xstr.

BTW,the switcher transistor only dissipates power when it is ON,when it's
off,it passes no current,thus no dissipation.That's why they use low ON-
resistance devices,to reduce dissipation inthe switcher xstr.

the two operations are very different and there is no simple analogy
between them.

Also,I suspect there haven't been many -worn out- main relays that
were due to contact wear,if any. Just cracked solder joints due to
aging of the solder alloy itself,or thermal cycling unrelated to
current flow thru the contacts.The current drawn by the main relay
is not enough to heat up those contacts,especially with the low
make/break duty cycle.

the contacts don't heat much when new, but after a while in
operation, even with spark protection, the contact points get eroded.
it may not look much to the naked eye, but reality is that the
actual surface contact area decreases - you can see all the little
hills & valleys that have been eaten away under a microscope. as the
current per unit area increases [same current divided by less area],
operating temperature increases.
Click to expand...

Yes,and for the -main relay- we're discussing,how often does the
make/break happen? Not anything like a automotive ignition doing it
thousands of times per second,nor is there a high current.
Click to expand...

well, you're right that the switch count is not high, but take one of
these relays apart, particularly if you get a chance to put it under a
microscope, and you'll see the contact erosion. also, it's switching
the fuel pump which has a fairly large coil in it....
Click to expand...

And the inductance of the long wiring between relay and fuel pump windings
helps limit that.

lackej1971 · Sep 24, 2004

I had a similar problem and you may want to check your timing belt. If your
car has jumped time, it will be hard to start at different times, but there
can be horerendous engine damage if the situation is not corrected in time
in the form of bent valves and wrecked pistons.

lackej1971

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